1. Field of the Invention
This invention relates generally to the tuning and operation of resonant electrical circuits and more particularly to an arrangement for solid state electronic controls to vary the capacitance in a series resonant circuit so that the resonant frequency of the circuit matches the imposed operating frequency.
2. Description of the Prior Art
Resonant circuits are variously used to store or modulate energy in magnetic fields, or to generate high voltages and electrostatic potential. Such circuits find use in radio technology, in electronic display technology, in induction heating systems and in power generation technology. Resonant circuits are extremely sensitive to frequency; component values must be closely matched to operating frequency or the resonant effect will be significantly diminished. Natural variation of component and circuit parameters due to aging, stray inductance, stray capacitance, circuit loading, or drifts in operating frequency present difficulties for keeping a resonant circuit tuned for maximum response.
Resonant electrical circuits contain, primarily, inductive and capacitive elements. The circuits are arranged so that energy is stored alternately in the magnetic field of the inductor and then in the electrostatic field of the capacitor. The stored energy shifts back and forth in a periodic fashion, ideally in a sinusoidal fashion. The resonant frequency depends on the values of the inductance and capacitance. Resonant circuits can be tuned by adjusting the values of the inductor and/or the capacitor. Variable capacitance can be used to tune a resonant circuit as is well known in radio technology and other arts. At power electronic levels, however, the radio techniques of varying capacitance become less practical.
A known technique to vary the capacitance is to add trim capacitors in parallel across the capacitive portion the circuit, as disclosed by Bolotski, et al. in U.S. Pat. No. 6,108,000. This technique generally has to be done with the circuit un-powered or as the capacitive element voltage crosses zero. These adjustments are limited to a discrete number of capacitor combinations available. Obtaining a range of resonant operating frequencies involves having a bank of many capacitors available and is difficult to coordinate with automatic control.
Lusher, et al., in U.S. Pat. No. 5,640,082 disclose a pulse width modulation (PWM) technique utilizing a solid state switch to dynamically vary the capacitance in a power filter arranged parallel to a DC load. This technique relies on one end of the switched capacitor being grounded. The pulse width modulation of the variable capacitor happens only on the positive phase of AC power input and is thus not suited by symmetry to use in shaping AC output waveforms.
Glaser, et al., in U.S. Pat. No. 5,399,955 disclose a technique utilizing pulse width modulation to dynamically vary the capacitance in a power filter which is arranged parallel to an AC power load. This technique utilizes two solid state switches and can be used in a symmetrical fashion to shape AC output waveforms. The solid state switches shown by Glaser, et al, introduce switching and resistive power losses that need to be minimized for use in resonant circuits. Resonant circuits are sensitive to all sources of power loss within the circuit. Resonant xe2x80x9cqualityxe2x80x9d factor xe2x80x9cQxe2x80x9d is defined as:
Q=(2*pi)*(peak energy stored per cycle)/(total energy dissipated per cycle).
Obtaining a high level of Q is an important efficiency and design feature of resonant circuits and unnecessary losses reduce the effective quality factor Q.
Glaser, et al., disclose no importance or restrictions on the timing of the pulse-width signals which enable capacitance to be added for the filter circuit operation. In series resonant circuits the voltages across the inductive and capacitive elements are naturally amplified to high levels and high levels of energy are stored within the circuit. For resonant circuit use it is important to prevent the trim capacitors from being added in parallel into the circuit at the time points when the voltage in the capacitor is different from the voltage of the circuit across the terminals to which it is being connected. If the capacitor at one voltage were allowed to connect in parallel with the circuit at a second, substantially different voltage, then large instantaneous xe2x80x9cshort circuitxe2x80x9d currents would flow between the capacitor and the circuit. Such xe2x80x9cshort circuitxe2x80x9d currents could easily reach destructive levels and damage equipment, and such currents would generate a broad spectrum of undesirable radio frequency noise.
Because resonant circuits are excellent narrow band filters centered on the resonant frequency, it is possible to stimulate resonant behavior of a series resonant circuit by the application of driving voltages using non-sinusoidal waveforms; despite the non-sinusoidal stimulation the resonant behaviors, current waveform, and energy storage patterns of the circuit remain predominantly sinusoidal at the excitation frequency. This feature may be exploited to allow a single sided DC voltage source to be used to stimulate the resonant circuit with rectangular pulses. Better, more sinusoidal waveforms are obtained if the excitation is symmetrical with respect to positive and negative portions of the cycle, and this may be accomplished by adding an H-bridge commutation circuit. The amplitude of the resonant response depends upon a complex interaction between the amplitude of the excitation and the relationship between the excitation frequency and the resonant frequency of the circuit. For a highly tuned resonant circuit a small misalignment between the excitation frequency (operational frequency) and resonant frequency leads to a substantial drop in resonant behavior and energy storage levels. Pulse width modulation techniques may be added to the rectangular pulses of the commutation circuit to provide control of the effective amplitude of excitation and thus resonant behavior. Tupper, in U.S. Pat. No. 6,051,959, shows an example of such an arrangement as applied to the excitation of the field of a high frequency alternator.
When a single sided DC supply is used with a commutation circuit to provide symmetrical excitation to a series resonant circuit, neither end of the capacitor is consistently at the ground reference. This introduces difficulties in activating solid state switches to control the variable capacitors by PWM techniques such as those cited.
Series resonant circuits generate high (amplified) voltages with a modest series current. This is useful in situations where driving current sources are limited and high voltages are tolerable or desirable. These high voltages (Q times the driving voltage) appear across both the inductor and the capacitor. Such high voltages create difficulties in measuring the zero crossing of the capacitor voltage in order to synchronize the pulse width modulation adjustment of capacitance. The presence of these high voltage signals among the usually low voltage control signals presents possible safety hazards that need to be minimized.
It is an object of the current invention to provide means to tune a series resonant circuit so that it""s natural frequency matches an imposed excitation (operating) frequency.
It is an object of this invention to provide for a continuously variable range of natural frequencies between a maximum and minimum while using a minimum of components and switches. It is a further objective of the current invention to provide for dynamic adjustment and tuning of a resonant circuit in a fashion that is suitable for automatic control. It is a further object to provide this continuous control while eliminating the possibility of creating short circuits and damaging currents during switching and while minimizing the generation of unwanted radio frequency noise.
It is an object of the current invention to minimize the need for high voltage signals to be present in the control system.
It is an objective of this invention to minimize resistive and switching losses.
It is a further object of this invention to provide means to control the amplitude of the resonant response of the resonant circuit powered by a single sided DC supply and to strive for a resonant response that approaches a sinusoidal waveform at the excitation frequency.
The objects set forth as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described herein below. The present invention includes a series resonant circuit that includes a series capacitive element and an inductive element in series with each other and together in series with a voltage driving source. The series capacitive element includes a base capacitor in parallel with a trim capacitor circuit. The trim capacitor circuit includes a capacitor in series with an switch circuit which can provide electronic control of the timing and direction of the current allowed to flow through the trim capacitor.
The switch circuit is powered or enabled by a floating voltage source and is controlled by phase modulated tuning signals from a control system. The phase modulated tuning signals are timed to allow conduction through the switch circuit in such a way that allows the trim capacitor to be added in parallel to the base capacitor for specific portions of the resonant response cycle and to be essentially disconnected from the base capacitor during the other portions of the resonant cycle. The phase modulated tuning signals are arranged to allow conduction through the trim capacitor to be initiated only by natural commutation when the voltage in the trim capacitor and the voltage in the base capacitor are essentially equal. This natural commutation prevents sudden discharge of current between the capacitors and helps prevent destructive current levels and the generation of undesirable radio frequency noise. Adjustment of the delay, or phase angle, of these tuning signals relative to the time of the zero crossing of the series resonant current allows the effective capacitance of the capacitive element, as seen by the resonant circuit over the course of a resonant cycle, to be varied in a continuous way between the value of the base capacitor and the value of the base capacitor plus the parallel trim capacitor. In turn, this adjusts the natural resonant frequency of the series resonant circuit.
The voltage driving source includes a single sided DC power supply, such as a battery, an H-bridge circuit to provide commutation of the polarity driving voltage at a desired operating frequency, and a control circuit to provide pulsewidth-modulation signals to the drive to control the effective magnitude of the excitation. The drive also includes arrangements to allow low-loss free-wheeling currents during unexcited portions of the PWM drive cycle. A current detector provides an indication of, at least, the direction of the total series current flow in the resonant circuit. A phase detection circuit provides a signal indicating the phase difference between the driving voltage and the total series current in the resonant circuit. A control system integrates the phase difference signal and provides an integrated error signal, which is used to control the timing of the phase modulated tuning signals and thus the capacitance of the capacitive element of the resonant circuit and thus the resonant frequency. The control circuit is arranged to stabilize when the driving voltage and resonant current are in phase, thus providing auto-tuning operation. The control circuit also adjusts the PWM circuit to maintain the amplitude of the resonant response at a desired point.